Power combiners and dividers are often used in high power microwave and RF amplifiers. While traditional power combiners are well known, the need exists for a low loss, small size, n-way (where n is any positive integer) planar power combiner with high adjacent port isolation suitable for uses such as in the IRIDIUM.RTM. low-earth orbit satellite cellular communication system.
Several well known power divider and power combiner topologies exist. The most common binary (2.sup.n -way) method to provide high isolation power combining is by combining 2-way power combiners to form a 2.sup.n -way power combiner. The problem with this method is high combiner loss from using multiple 2-way power combiners or dividers. The power loss in this type of combiner or divider is proportional to the number of levels (the number of times two ports are split or combined) needed to realize the combiner.
Another method is to use the well-established Wilkinson n-way power combiner. A disadvantage of this method is that, as n gets large, the output line impedance becomes too high to be realizable. For example, consider a 4-way power combiner or divider requiring a line impedance of 100 ohms. Current n-way power dividers are designed to match n impedances directly to the 50 ohm input. For example, to split or combine a signal n-ways, the n-way paths must be transformed from 50 ohms to 50 times n ohms. For a 6-way combiner/divider, a transformation from 50 ohms to 300 ohms is required. Generally, a quarter wave line of an impedance of [50*(50*n)].sup.0.5 is used for transforming from 50 ohms to 50*n ohms. For the 6-way, a 123 ohm quarter wave line is required, but not feasible in most microstrip substrates because of its extremely small line width.
A structure referenced in the article "An N-way Broadband Planar Power Combiner/Divider" by Yau, W. and Schellenberg, J. M., Microwave Journal, November 1986 transforms a 50 ohm input to a total n-way load of 50/n using a Dolph-Chebyshev tapered transmission line. The line is segmented to form n output ports. Isolation resistors are placed between the segmented output ports to achieve isolation between the output ports. Disadvantages inherent in this structure include the following:
a. The structure cannot accommodate large power amplifier chips because each segmented output port relies on the coupling between lines to keep the integrity of the Dolph-Chebyshev tapered line;
b. Isolation resistors limit the structure's highest operating frequency and power handling capability;
c. If isolation resistors are removed to accommodate higher operating frequencies and power handling capability, the structure will not lend itself to soft failures; and
d. The structure voltage standing wave ratio (VSWR) tolerance to variations in loads is very poor.
What is needed is a planar n-way power combiner or divider that accommodates larger amplifier chips, provides good port-to-port isolation, and is not limited to low microwave frequency realizations.